1. Drag and drop a folder of jpeg or tif images.
2. BookCrop overlays them into one preview.
3. Draw a rectangle and click "crop" to crop all images.
(4. or Ctrl+draw another rectangle to the right of the first to crop split each image to two files)

Download and unzip BookCrop.
Download and install the latest GraphicsMagick (the Q8 version is faster).
Download libjpeg-turbo-1.4.90-gcc.exe (for 64bit get gcc64.exe) or newer and
unzip the exe (with 7-zip) and browse to the \bin subfolder and
copy jpegtran.exe and libjpeg-62.dll and place next to BookCrop.exe

Notes

Use BookCrop on folders with right and left pages before ScanTailor when book scanning.
BookCrop is like Briss but for jpeg files.
BookCrop crops all and only jpeg or tif files (whichever is first found) in the dropped folder.
Other files and subfolders are ignored.
Change threshold if overlay is too dark or light.
Drop a single jpeg/tif for quick overlay preview that refreshes when the slider is moved.
Advice: Use crop to subfolder, so you can redo if you crop too much.
Preview works well only if all input jpegs are equal in pixelsize.

Made in Autohotkey by nod5 as Free Software GPL3
Tested in Win7 x64. The zip contains binary and source.